For years, it has been hammered into our brains that the best way to prevent deaths from breast cancer was to find it as early as possible and treat it right away. That led to a set of recommendations for routine mammography screenings, and that in turn led to a lot of breast cancer being discovered while it was still highly treatable. All good things, to be sure. But now it seems we're finding breast cancer too early.

A new study appearing in today's Annals of Internal Medicine has found that between 15 and 25 percent of breast cancers discovered by mammograms would not have caused a woman any problems during her lifetime. This problem is known as "overdiagnosis," a strange term, which Dr. Otis Brawley, chief medical officer of the American Cancer Society, explained to CNN in this helpful way:

[I]t refers to a tumor that fulfills all laboratory criteria to be called cancer but, if left alone, would never cause harm. This is a tumor that will not continue to grow, spread and kill. It is a tumor that can be cured with treatment but does not need to be treated and/or cured.

Overdiagnosis has already been seen in treating prostate cancer and to some extent in thyroid, lung, and other cancers. In the case of breast cancer, because we can't yet tell which types of the cancer spread and kill and which don't, we need to treat any tumor that's found as if it's deadly. That means surgical removal and sometimes radiation or chemotherapy.

There have been other studies that looked at the phenomenon of overdiagnosis, but estimates of how prevalent a problem it is have varied. This new study is particularly robust because it used data collected in Norway during the introduction of a screening program that took place over the course of a decade. This allowed researchers to compare breast cancers from counties where screenings were offered against places where they weren't yet offered. Researchers ended up analyzing some 40,000 breast cancer cases in total, including 7,793 cases where cancer was detected after routine screenings started. They found that between 1,169 and 1,948 of those women were overdiagnosed and received unnecessary treatments. Because Norway offers women screenings every two years between the ages of 50 to 69, it's estimated that the number of overdiagnosed cases would actually be even higher in the U.S., where we begin routinely screening women at 40 instead of 50.

So what exactly are we supposed to do about this? Well, mainly just be informed, which is medicalese for "be freaked out with no way to help yourself." It's clearly a problem, but there is no clear solution. If you take a more macro view, theoretically we ought to stop screening women so early (unless they have a family history of breast cancer), and we ought to be less aggressive in treating certain breast cancers. But that idea obviously becomes problematic when you try to practice caution on any given individual. Back in 2009, when a task force made the bold suggestion that we not start routine mammography until age 50, people went nuts. Some with good reason: they had a mother, sister, aunt, friend, grocery cashier, etc. whose life had been saved because of a mammography before 40. And, of course, there are many cases where early screening is an actual life saver, but for all of those cases there are these other cases of women being overdiagnosed and getting unnecessary surgery or chemotherapy, which is a highly undesirable medical outcome too.

The biggest problem is that although we have the technology to find cancer, we don't yet have the ability to discern what kinds of cancers are the really dangerous ones. From an individual physician's standpoint, this means they have to act in their patient's best interest and assume the worst and treat accordingly. Part of the responsibility lies with radiologists, who could raise their threshold for reporting abnormalities spotted on mammograms, but that has its issues too. In an editorial that accompanied the study, the doctors suggested a "watch-and-wait" approach could also be implemented, rather than taking immediate biopsies, but they also admitted that might be a "tough sell" to patients and doctors. Ding, ding ding!

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Dr. Mette Kalager, the leader of the study, says women really need to be educated about the fact that mammograms can pick up cancers that will never be life-threatening. Of course, even knowing that, what are the chances you'll either hold off on getting a mammogram or get one, find a tumor, and then be like, "Nah, it's cool. I'm going to take my chances with this one, doc." That's why Kalager says it's more about understanding that you'll have to be ready to deal with the ramifications of screening before you do it:

Once you've decided to undergo mammography screening, you also have to deal with the consequences that you might be overdiagnosed. By then, I think, it's too late. You have to get treated.

Yes, you do. At least Dr. Brawley of the American Cancer Society thinks so. He concludes that while we do need to understand the pitfalls of overdiagnosis, we need to keep getting treated as though our cancer is the dangerous kind:

A woman who is diagnosed with cancer should get it treated appropriately. Unlike the case of prostate cancer, observation of an early breast cancer mass is not appropriate therapy at this time; we have definite proof that, while we may treat some women who do not need to be treated, we definitely cure many women who need to be cured.

Fair enough. Now we know. Perhaps someday in the future, there will be a little robot that slips in through your nipple (eek) and can immediately identify the kind of enemy that's lurking within and recommend the appropriate treatment. Then we might learn to be totally casual about tumors growing inside our mammary glands. But for now, in addition to all the existing anxiety about whether we have breast cancer secretly invading us as we go about our business, we can add the fear that we will someday overreact to our cancer and get chemo we don't need.